Since polyimide resins have excellent heat resistance and are comparable to other plastic materials in view of mechanical, electrical and chemical properties, they are often used as insulating materials such as electric appliances. Printed wiring boards (PWB), flexible printed circuits (FPC), tape automated bonding (TAB), etc. are manufactured by applying photoetching to a copper layer formed on the polyimide resin.
Copper-polyimide substrates as the material used for such PWB, FPC and TAB have generally been prepared by a lamination method of appending a polyimide resin and a copper foil by means of adhesives . However, in a substrate prepared by the lamination method, impurities such as chloride ions or sulfate ions are adsorbed to an adhesive layer present at the boundary between the copper layer and the polyimide resin of the substrate upon etching treatment for the copper layer or peeling treatment for the photoresist, which may lead to a trouble such as insulation failure in a case where distance between circuits formed on the substrate is particularly narrow. In order to overcome such a drawback, there has been studied a method of forming a metal layer directly to the surface of the polyimide resin not by way of the adhesive, etc.
As the method described above there has been known a method of depositing a metal layer directly to the surface of the polyimide resin by means of sputtering, ion plating, vapour deposition, etc., a method of coating a solution of a polyamic acid as a precursor for a polyimide on the metal layer and, thereafter imidizing the polyamide to obtain a substrate or a method of forming a metal plating layer on the surface of a polyimide resin by means of electroless , plating.
Among the methods described above, in the method of depositing the metal layer directly to the surface of the polyimide resin, for example, by means of sputtering, ion plating and vapour deposition, since residual stresses are formed in the copper layer deposited on the surface of the polyimide resin, stresses are concentrated to the boundary between the polyimide resin and the copper film layer when a heat treatment is further applied to the substrate to remarkably reduce the adhesion of the copper film, which may be some time lead to the peeling of the film.
Further, a polyimide resin deposited with a copper film by means of sputtering, ion plating or vapour deposition has often been heated for improving the adhesion of the metal film layer, but a problem is also brought about that copper diffuses into the polyimide resin and deposited as copper oxide particles, thereby remarkably deteriorating the electric property and mechanical property of the substrate.
In order to overcome these problems, it has also been proposed a method of forming an intermediate chromium or nickel layer between a polyimide resin and a copper film to obtain a three-layered structure, applying heat treatment to the stricture, thereby absorbing the stresses concentrated to the boundary in this intermediate layer to prevent reduction of adhesion and peeling of the copper film, as well as to prevent the deposition of copper oxide caused by the diffusion of copper in the polyimide resin. However, in a case of forming circuits by using the substrate obtained by the method and applying patterning, for example, by means of subtracting method, since the metal layer formed on the polyimide resin contains metals such as chromium or nickel in addition to copper, other metals such as chromium or nickel cannot be removed completely but remain as they are when treated with a standard etching solution prepared for the removal of copper, failing to obtain satisfactory circuit formation.
In addition, if chromium or nickel is contained in the metal layer, electrical conductivity is poor as compared with the case that the layer is formed out of pure copper and, if such a substrate is used as electronic material part, reliability is remarkably reduced. Further, even with such a method, when TAB is manufactured based on the resultant substrate, the adhesion strength of the metal layer is utmost 4 lb/in, which is less than 6 lb/in required so far for TAB or printed wiring board.
Further, among the substrate forming methods described above, a method of coating a polyamic acid solution as a precursor of the polyimide on a metal film, and then polyidizing imidizing the same thereby obtaining a copper-polyimide substrate without using the intermediate adhesive layer has been disclosed in U.S. Pat. Nos. 3682960, 4148969 and 4496794, etc. However, in the copper-polyimide substrate obtained by the method, the resin portion of the substrate shrinks remarkably since dehydrating condensation also occurs upon polyimidization of the polyamic acid, which makes it extremely difficult to provide smoothness for the substrate and the substrate lacks in dimensional stability.
Furthermore, the polyimide resin portion in the substrate obtained by the method is remarkably lower in the mechanical strength as compared with the polyimide resin usually supplied in a film-like shape. The reason is because the chemical structure of the polyimide resin obtained by the above-mentioned method is different from that of the polyimide resin supplied in the film-like shape and, although the bondability with the metal layer can be improved, mechanical property such as elongation is poor. Furthermore, in a case where the polyamide acid is coated on the copper film, since a copper portion reacts with amide groups and diffuses into the resin portion, electric property of the substrate is deteriorated and reliability as electronic material is neither sufficient.
Further, the third method for obtaining the copper-polyimide substrate is a method of applying metal coating to the surface of the polyimide resin by means of electroless plating. In this method, etching treatment is usually applied to the surface of the polyimide resin, thereby providing hydrophilic property, adsorbing thereon palladium, etc. as a catalyst and then applying electroless plating.
U.S. Pat. No. 3767538 describes a method of producing a substrate applied with a metal coating by means of electroless plating. The feature of the process resides in applying etching to the surface of the polyimide resin using a solution of sulfuric acid and hydrochloric acid, or mechanically impinging fine particles such as sands to the surface, further, applying etching if required by means of sodium hydroxide, heating the surface to remove water content, and then immersing it into a colloidal palladium bath, thereby effecting a catalyst activation treatment. Subsequently, a silver plating layer having steam permeability is continuously formed on the surface of the polyimide resin by means of electroless plating, heating the substrate to 150.degree. C. for removing water content and, thereafter, applying electrolytic copper plating to the silver film thereby obtaining a copper-polyimide substrate.
However, nothing is reported for the value of the adhesion of the metal layer to the substrate manufactured by the method and, further, since the silver film is directly formed on the surface of the polyimide resin in this method, there is a possibility that insulation failure may be caused between circuits due to silver migration under a high temperature and a high humidity, thus defectively lacking in the reliability in a case of using the TAB tape, etc., manufactured by using the resultant substrate as electronic parts.
U.S. Pat. No. 3573937 suggests another method of forming a metal layer by applying electroless plating to the surface of a polyimide resin. The method resides in applying a catalyst to the surface of a polyimide resin, then forming a plated resist layer, applying nickel-phosphor alloy electroless plating, heating the substrate at a temperature of 190.degree. C. for strengthening the bonding between the plated layer and the polyimide resin and, subsequently, electrolytically plating copper or applying or soldering copper thereon, to obtain a substrate. However, the maximum adhesion of the metal layer is reported to be 5.0 lb/in according to the non-standard test. While the literature suggests nothing about the stability of the adhesion against thermal shocks such as soldering, there is a possibility of causing problems regarding this.
Japanese Patent Laid-Open Sho 63-259083 discloses a method of applying electroless nickel or cobalt plating partially or entirely to the surface of a polyimide resin and applying electrolytic copper plating thereon, thus forming a metal film having a great bondability capable of withstanding thermal shocks such as soldering on a polyimide resin.
The feature of the methods resides in applying etching to the surface of the polyimide resin with an aqueous solution containing an amine of H.sub.2 N(CH.sub.2).sub.n NH.sub.2 (n represents an integer between 2 and 6), an alkali metal hydroxide and an alcohol of a structure soluble in water, providing a catalyst, applying electroless nickel and cobalt plating and, finally, applying electrolytic copper plating.
The adhesion of the metal layer in the disclosed example of the substrate obtained by the method is 10.0 lb/in according to IPC-TM-650 Method 2.4.9. As Received Method A and 5.0 lb/in according to IPC-TM-650 Method 2.4.9. After Solder Float Method C, by which the adhesion satisfactory to some extent can be nickel and cobalt in the substrate obtained by the above-mentioned method could be removed finally by means of an iron chloride etching solution, if the substrate is utilized for a TAB tape, with narrow width and interval of lead, the copper lead cannot maintain the shape till the complete removal of the nickel and cobalt layer due to the difference of the solubility of nickel and cobalt relative to iron chloride and, accordingly, reliability is poor when the TAB tape is assembled as an electronic part. Further, in a case of forming a copper layer by electroless copper plating directly to the surface of the polyimide resin by the above-mentioned method, it has been shown that the copper layer is peeled off in IPC-TM-650 Method 2.4.9. After Solder Float Method C. Thus, at present, it has not yet been actually established a technique capable of directly forming a copper layer having adhesion capable of withstanding thermal shock such as soldering partially or entirely on the surface of the polyimide resin substrate.
Further, in a case of using a substrate obtained by applying an electroless plating to the polyimide resin as described above, applying patterning to form a copper layer of a desired width and then applying plating to the copper layer by using a solution containing a free cyan or cyan compound thereby preparing a wiring board, there has also been found a problem that the copper layer is peeled off from the surface of the polyimide resin.
On the other hand, recently with elevation of the integration degree of IC, LSI and the like electronic devices, it has become expected that a high heat energy is to be irradiated from IC, LSI or the like device to a substrate for a long period of time when such a device is fitted to the substrate. Under the situation, the substrate is required to have a heat-stability stability as its characteristic when a high heat is imparted to the substrate for a long period of time.
The reliability of a substrate with respect to the high-temperature resistance characteristic thereof under the condition of a high temperature ambient for a long period of time may often be determined by examining the change, if any, of the characteristic values of the substrate, after the substrate to be examined has been stored in an atmosphere of 150.degree. C. for a period of 1000 hours.
Regarding the adhesion strength of the metal layer after the above-mentioned heat-resistance test, for example, in the case of TAB, a critical value of 1.0 lb/in or more is required. However, it has been found that the adhesion strength of the conventional substrate is lowered to 0.5 lb/in after the heat-resistance resistance test, which means that not only the substrate could not satisfy the above-mentioned indispensable value when it has been exposed to a high temperature ambient for a long period of time but also the metal layer would be peeled off from the substrate because of the lowering of the adhesion strength of the layer to the substrate when IC, LSI or the like device is fitted to the substrate and a high heat energy to be irradiated from such device is imparted to the substrate for a long period of time.